1. Field of the Invention
This invention relates generally to ceramics and more particularly to the efficient production of high alumina bodies with very smooth surfaces. Such bodies, particularly when thin and flat, are especially valuable as substrates for the construction of electronic devices adapted to service in high reliability applications.
The widespread commercial use of thin film devices is dependent on the availability of substrates with high surface smoothness (particularly freedom from surface voids), uniform small grain size surface microstructure, high flatness, and high density, so that the effectiveness of the electrically controlling components is not vitiated by irregularities of the surface on which they are deposited. For example, a tantalum capacitor in a thin film microcircuit might have a thickness of only a few hundred Angstroms. If the substrate surface had a roughness of 0.5 micron, which is 5000 Angstroms, a tantalum layer across such a variation in the surface might well be discontinuous. Even if the tantalum film were in fact continuous across a surface variation, the divergence of the tantalum from its expected planar shape might significantly vary its capacitance from the expected value, leading to an ineffective circuit. Specifically, surface roughness of no more than 25 nanometers (nm) is desirable, although in practice roughnesses up to about 100 nm can be tolerated in many cases.
In microstrip applications, where the substrate is an active part of the circuit, porosity in the bulk of the substrate, which affects its dielectric properties, plays a major role in the reliability of the circuit. Uniform pore size distribution in the substrate is essential for its use in the gigahertz frequency range.
2. Description of the Prior Art
When considering means of attaining adequate surface smoothness on alumina components for electronics applications, there is an important practical distinction between essentially single crystal forms of alumina such as ruby and sapphire and the much more common polycrystalline bodies in more general use. Single crystal forms of alumina must normally be grown from melts in shapes substantially controlled by the natural characteristics of the crystal itself. The crystals are then machined to the shape desired for use, and the surfaces can be polished to whatever surface smoothness is required. For these and other reasons, single crystal forms of alumina are relatively expensive.
Because of their lower cost, polycrystalline alumina bodies are much more commonly used. Such bodies can be made in almost any shape desired by pressing or casting alumina powders mixed with appropriate binders well known in the art, then firing the pressed cast bodies to sinter the powders into a dense and coherent polycrystalline body. If care is taken, as for example is taught in U.S. Pat. No. 3,678,923 to Stetson and Gyurk, hereinafter cited as Stetson, these polycrystalline bodies often can be used in the as-fired condition. If greater smoothness is needed, it can be obtained by lapping and polishing techniques, as reviewed by John B. Snook, "As-Fired vs. Lapped-and-Polished Substrates for Thick- and Thin-Film Hybrid Circuits", Microelectronic Manufacturing and Testing, October 1983. Despite the ability to achieve average surface smoothness on polycrystalline substrates as low as 12 nm, as shown in Snook Table 1, exposure of relatively large microvoids on the surface is still common, as shown in Snook FIG. 5, and capable of causing the sort of difficulties already discussed. Furthermore, surface polishing can not remove subsurface pores, and the lapping and polishing processes can be slow and expensive.
The Stetson reference already cited describes the last significant advance in preparation of smooth electronic substrates by direct firing known to the applicants before their own invention. This patent also contains, at column 2 line 43 to column 3 line 8, substantial detail about the meanings and techniques of surface flatness and surface roughness measurements, which are hereby incorporated by reference. In the instant application, as in Stetson, surface roughness measurements will be given as measured according to the center line average or arithmetic average (AA) method. The measurements of surface roughness described herein were carried out with a commercially available instrument, the Tallysurf, available from Rank-Taylor-Hobson of Leicester, England, which as a precision of .+-.2.5 nm.
The Stetson specification teaches the use of extensive mixing in organic solvent, along with other expedients, to achieve alumina powders which can be tape cast to give continuous thin substrates with surface finishes as fired between about 50 and 90 nm. Although no explanation is offered for such a relationship by either Stetson or the instant applicants, the graphs disclosed by Stetson, particularly Stetson FIGS. 5A and B and 6A and B, are strongly suggestive of an asymptotic mathematical function, with the asymptote at about 50 nm. With or without a theoretical explanation, the data shown by Stetson indicate very strongly that 47.50 nm is the practical limit of surface finish achievable by the use of long mixing and/or low sintering temperatures, the primary techniques taught by Stetson for achieving good surface finish. Particularly significant in this regard is the observation (in column 7 lines 59-65 of the Stetson specification) that doubling the longest mixing time shown on the Stetson Figures did not result in any measurable increase in the surface area (and possibly in surface finish as taught by Stetson) of the milled alumina powder.
A characteristic of the processes disclosed by Stetson and the other prior art is a requirement of the use of size-graded or size-ranged mixtures of alumina powders in order to obtain non-porous final fired bodies. As expressed by Stetson at column 3 lines 65-66, "Little particles must be available to fill in the holes between the big particles."
Another method of obtaining a fine grain dense aluminum body is by hot pressing as described by W. M. Wheildon in Modern Materials, Vol. 2, p. 111 (published by Academic Press, 1960). By grinding and polishing such products, very smooth average surface finishes can be obtained. However, the grinding and polishing operations result in grain pullouts and thus defects in the surface at least as large as the grain size of the body. In summary, the prior art teaches how to obtain by direct firing ceramic substrates with water-impervious surfaces having a roughness of 50 nm. However, the substrates obtained have a relatively low density of 3.7 megagrams per cubic meter (Mg/m.sup.3) compared to a theoretical density for Al.sub.2 O.sub.3 of 3.97 Mg/m.sup.3 (Stetson column 12, lines 35-40).